1. The tetrapeptide Ala2-Nph2 (where Nph = p-nitrophenylalanyl) is treated by porcine pepsin to study the mechanism of aminotranspeptidation reactions. 2. The major initial product is Ala2-Nph and the major transpeptidation products are Nph2 and Nph3 accompanied by some Nph, a little Nph4, Ala2-Nph3 and Ala2-Nph4. 3. Oligomers of Nph greater than tetramers are formed near the end of the reaction. 4. In presence of [3H]Nph, no incorporation of Nph into the transpeptidation products is observed. 5. 18O-labeling shows extensive incorporation of 18O atoms from [18O]water in the carbonyl oxygens of Nph residues.